In recent years laser printers have come into widespread use because of their ability to rapidly and quietly print high quality images at a reasonable price. A typical laser printer prints two dimensional arrays of dots having a resolution of 300 dots per inch both vertically and horizontally. This is sufficient resolution to print letters, legal documents and other documents with what is known as letter quality
In operation a laser printer is connected to a host computer system through a standard communication path that connects to either a serial port or a parallel port. When operating in a normal typewriter mode or character mode a laser printer is reasonably fast and efficient. A page of letter and number characters can be printed in 8 to 15 seconds. This is fast enough for most business applications.
However, laser printers are capable of printing arbitrarily selected graphic data as well as predefined characters. Because of the high quality of images printed by laser printers, the demand for printing graphic data is increasing rapidly. Persons using a laser printer to print graphic data quickly realize that printing graphic data takes much longer than the printing of character data. For example, it typically takes up to 2.49 minutes to print a full page of bit mapped graphic data when the printer is connected to the host computer through a parallel port and up to 6.94 minutes when the printer is connected through a serial port.
The much longer time required to print graphic images does not result from an inherent limitation in the printer. The printer is capable of printing graphic images as fast as it prints character images. The increased print time for graphic images is caused by a bottleneck in the communication path between the host computer and the printer.
When printing the letter "A", the host sends to the printer a single 8 bit code that represents the letter "A". The letter A is then printed as a rectangular matrix of tiny dots. The dots are printed to turn the paper black along the lines defining the letter and the dot spaces surrounding the strokes of the letter are not printed so the paper is left white.
A typical print character such as the letter "A" is defined by a matrix of dots having 30 dots per row and 50 rows for a total of 1500 dot positions. The printer has an internal computer and a data store that stores one bit of information for each dot position in the dot pattern matrix for each character. Each bit indicates whether the corresponding dot position is to be printed black or left white.
Thus, in response to a received 8 bit command to print the letter "A", the printer's internal computer reads from the data store the 1500 bits which define the print image for the letter "A" and commands the print mechanism to print or not print each corresponding dot position, depending on the state of the data that is read from the store. Since a similar 1500 bits must be stored for each different letter or character that can be printed, the font memory or store that stores this data must be quite large.
Since a single 8 bit command causes the printer to print an image pattern having 1500 dots, relatively little data passes between the host computer and the printer when printing ordinary text or character data. The relative slowness of the communication channel between the printer and the host is of no particular consequence. The printer is internally designed to very rapidly access the font image data that corresponds to character print commands and a page of data can be printed quite rapidly.
However, when the printer is switched to graphics mode, all of the data defining the tiny dot patterns that are printed must be sent through the communication path between the host computer and the printer. This path then becomes a communication bottleneck and greatly slows the rate at which graphic data can be printed.
In an attempt to circumvent this communication bottleneck, one system has been developed which avoids the normal serial or parallel print command communication path. Some printers have a communication access port known as a video port that provides direct electronic access to the printer print mechanism. The developed system provides high speed data communication directly from the host computer to the video port.
While use of the video port to communicate data is believed to increase the speed of graphic printing, this technique has not been widely accepted. In order to use this technique, the customer must use a screwdriver to remove a cover plate from the printer and then install a special printed circuit board into the video port which lies behind the cover plate. A special circuit and special software must also be installed at the host. This method has met with much customer resistance. Furthermore, the video port was designed primarily as a manufacturing test facility and not for the purpose of convenient user access. Such a port has not even been provided on some recently introduced models of laser printers. A significant need still remains for a satisfactory means of rapidly communicating graphic data between a host computer and dot pattern image printer.